1. Technical Field
The present invention relates generally to a nondestructive testing apparatus and method of nondestructive testing. More particularly, it relates to an apparatus and method for nondestructive testing of an object surface to determine the location of surface and subsurface flaws or other features using transient infrared thermography.
2. Related Art
The presence and location of both surface and subsurface flaws can be determined by various methods or techniques, including through the use of transient thermography. Transient thermography techniques generally utilize variations in the transfer of heat through an object over time to identify surface and subsurface flaws.
One transient thermographic method records the temperature rise of each resolution element by capturing a series of image arrays using an infrared camera while heating the surface of an object to be tested, and analyzing the temperature rise as a function of time to identify whether a linear temperature increase versus the square root of the heating time, occurs. Such behavior is reported to be indicative of the existence of a flaw within the object. This method is also reported to be of limited applicability for testing objects with complex geometries, or those which are otherwise subject to non-uniform heating transfer through the object.
One method of transient thermography comprises heating the surface of an object and analyzing individual infrared images of the object surface as a function of time. Flaws, particularly subsurface flaws, may be identified in the images by “hot spots” or regions of higher intensity infrared radiation because the rate at which heat transfers through the flaws is less than the rate at which it transfers in regions which do not include flaws. One limitation associated with this method is that the transient thermographic analysis is reduced to analyzing individual images which each represent a single point of time. This technique is of limited applicability with operators because of the possibility of missing a flaw indication because it requires analysis of multiple images which may have only minor variations to identify a flaw. Further, it is not easily adaptable to automation.
Another reported thermographic NDT technique utilizes a video replay of the recorded images and visual identification of intensity variations or bright spots by an operator to detect flaws. This technique is also not readily automatable. Further, it is highly operator dependent, and defects may easily be missed if the operator's attention is drawn away while observing the video. This technique also generally does not provide quantitative information about the location and or depth of the flaw.
A method of transient depth thermography is described in U.S. Pat. No. 5,711,603 to Ringermacher et al. This method involves heating the surface of the object and recording successive thermal images of each resolution element or pixel of the surface over a period of time. The contrast of each pixel is determined for each successive thermal image of the surface by determining the mean pixel intensity for that thermal image and subtracting the mean pixel intensity from the individual pixel intensity. The location of a flaw within the object is based upon changes in the pixel contrast.